An autofocus arrangement is known from DE 41 31 737 C2 (corresponds to U.S. Pat. No. 5,288,987). It is used for a stereomicroscope having a main objective. With this autofocus system, a projection beam path is generated by a projection optical system, the projection optical system comprising a laser diode, a collimator lens, optionally an attenuator, and a cylindrical lens, arranged one behind another. With this, the image of a strip-shaped marking is generated in a focal plane, said image in turn being imaged onto the specimen surface by means of a further lens, a deflection element, and the main objective of the stereomicroscope. The projection beam path thus generates a strip pattern, constituting an autofocus pattern, on the specimen surface. By means of a further deflection element, a portion of the observation beam path that contains light of the projection beam path reflected from the specimen is coupled out and conveyed to a light-sensitive, spatially resolving position detector made up of multiple individual sensitive elements and constituting an autofocus analysis unit. Here the projection beam path corresponds to the irradiation beam path mentioned initially, and the coupled-out observation beam path corresponds to the detector beam path mentioned initially.
With this known autofocus system, defocusing results in a lateral shift of the image of the strip-shaped marking on the position detector; that shift is registered via the autofocus analysis unit and used as a control signal for a focus motor for refocusing. The principal criterion in the context of this known autofocus system is the fact that here the projection beam path to the specimen proceeds entirely outside the observation beam paths, and interfering reflections at the objective into the observation beam paths, caused by the projection beam paths, are largely precluded. For autofocusing, the intercept distance of the objective being used is adjusted in motorized fashion, or the entire stereomicroscope is moved along the optical axis in motorized fashion.
Another autofocus system is known from EP 1 241 506 A2 (corresponds to US 2002/0186464). Here a video camera that is present on a surgical microscope for documentation purposes is used to ascertain the correct focus position of the microscope. The camera comprises an image sensor and a signal processor unit, the latter ascertaining the optimum focusing, for example, using the contrast method. By means of an interface in the camera and a further interface in the microscope, the control signals generated in the camera are transferred, for focusing, into a control unit of the microscope; said unit applies control to corresponding actuating units in the microscope, which modify the objective focal length for autofocusing. Except for the aforesaid video camera, a separate measurement module is not used in this autofocus system.